1. Field of Invention
The present invention relates to the field of linear measurement, specifically to optical methods of measuring microscopic distances between objects, e.g., between a magnetic read/write head and a magnetic storage disk, that participate in relative motion.
2. Description of Prior Art
During the operation of a computer disk drive, a read/write (transducing) head is supported on a thin cushion of air created by an adjacent magnetic disk, which is rotating at a high speed. The head must not "fly" too high or too low above the disk, but within a narrow elevation band in order to perform properly. Hence, it is necessary to be able to measure accurately the distance between a head and a disk for purposes of design and manufacturing. At the present time, flying-height testers, employing optical methods, are commonly utilized for this task. As with many other accurate measuring instruments, calibration of these devices is required.
The aforementioned flying-height testers measure the distance between facing, essentially parallel plane surfaces of a transducing head and a reference body representing a magnetic disk and composed of a homogeneous transparent dielectric. To measure this distance, electromagnetic radiation, such as visible light, is directed through the transparent object into the gap between the plane surfaces. The intensity of radiation reflected from the gap is related to the spacing between the surfaces and this spacing is determined from the measured intensity by means of a calibration table. However, optical parameters, such as refraction indices, of the objects bounding the gap are not known sufficiently well to allow the use of theoretically derived calibration tables. Therefore, a calibration procedure which determines a relationship between the intensity of reflected radiation and the true separation of the aforementioned surfaces is required.
In U.S. Pat. No. 4,624,564 to Robert Dahlgren, 1986, a "gap standard" for calibration of flying-height testers is proposed. The gap standard is a layered structure which is manufactured by depositing and etching layers of reflective and transparent materials on a substrate, using successive vacuum deposition and chemical etching operations. The standard simulates a gap of a precisely known height.
However, the manufacturing technology of the gap standard makes it impractical to produce the standard from the same materials as those of the objects used in the actual measurement. Substitutes for the original materials have to be employed, thus undermining the validity of calibration, especially when the optical properties of the original materials are not fully known. Moreover, since a single calibration standard simulates only one gap width, a large set of standards has to be manufactured and measured to obtain a calibration table. Furthermore, vacuum deposition and chemical etching technologies, used to manufacture the gap standard, render it relatively expensive.
Another calibration apparatus, disclosed in U.S. Pat. No. 4,681,451 to John Guerra et al., is shown in FIG. 1. A single-point contact is produced between two objects made of the same materials as those used in the actual measurement. The apparatus comprises two objects, one of which has a flat surface 18, while the other utilizes a convex surface 12 with a large radius of curvature. Surface 12 diverges predictably from surface 18 in the vicinity of the single-point contact. A width h of the gap between surfaces 12 and 18 at a point A is related through the predetermined geometry of surface 12 to a horizontal offset D. Offset D is the horizontal distance from the point of contact between surfaces 12 and 18 to a vertical line which goes through point A. Thereby, a range of known spacings is produced around the location of the aforementioned point of contact, and a calibration table can be obtained from the two-dimensional intensity pattern formed around this location.
However, this apparatus has several significant disadvantages. In reality, the single-point contact never occurs. Instead, the contacting objects deform under pressure in a way which is difficult to predict with sufficient accuracy, rendering the calibration relationship, which is based on the predetermined geometry of surface 12, invalid. Furthermore, liquid contaminants, such as water, accumulate inside the gap and are retained by a surface-tension force, thus making calibration below 25 nanometers (1 nm=1.times.10.sup.-9 meter) impossible.